Petroleum geology and oil exploration often apply reflection seismology to estimate underground features and thereby model the properties of a subsurface earth volume. A seismic source provides acoustic waves (i.e., “sound,” vibrations, or seismic waves). Subsurface features reflect the acoustic waves. A measurement of the time interval between transmission of the acoustic input and its arrival at a receiver enables estimation of the depth of the feature that reflected the sound or vibration.
When an acoustic wave impinges a boundary between two different subsurface materials that have different acoustic transparencies and acoustic impedances, some of the energy of the acoustic wave is transmitted or refracted through the boundary, while some of the energy is reflected off the boundary.
Depth conversion, as a part of the reflection seismology process and analysis, converts the acoustic wave travel time to actual depth, based at least in part on the acoustic velocity property of each subsurface medium (e.g., rock, sediment, hydrocarbon, water). This depth conversion makes possible a model. The model may have depth and thickness maps of subsurface layers interpreted from the seismic reflection data, and in turn the model enables volumetric evaluation of hydrocarbons, e.g., gas or oil, in their natural place.
In reflection seismology, the collected experimental data may be well data points, recorded seismograms, and so forth, and the desired result is the model of the physical structure and relevant properties of the subsurface earth volume. Models worked up from reflection seismology are usually very sensitive to small errors in the collected data or errors in the processing of the data. Such models that interpret experimental data collected from reflection seismology are often tentative and not very robust. Two specific sources of inaccuracy in modeling a subsurface volume from reflection seismology data are outlier values in the data and the influence of layer boundary conditions, i.e., uncertainty in the location of the tops and bases of horizons.